Image of a rosehip neuron (top of image) connected to a pyramidal neuron (bottom of image).

Tamás Lab, University of Szeged

An international team of 34 scientists has identified a new type of brain cell in humans not found in other well-studied species. The discovery of “Rosehip” neurons, published today in the journal Nature Neuroscience, raises a number of questions: How does it influence human behavior and experience? How does it differentiate us from other species? Can it be found in primates and other cognitively advanced species? But there is one issue this discovery highlights immediately: there’s a neuron in human brains that is missing from the brains of mice and other animals used to model human brains in experiments. Does this mean current animal models yield distorted results? “If we want to understand how the human brain works, we need to study humans or closely related species,” says Trygve Bakken, co-author of the paper and a neuroscientist at the Allen Institute for Brain Science.

The flow of info

Rosehip neurons are inhibitory neurons that form synapses with pyramidal neurons, the primary excitatory neurons in the prefrontal cortex.“We all have inhibitory neurons and excitatory neurons,” says Bakken, “but this particular type of inhibitory neuron is what’s new in this study. It's special based on its shape and its connections and also the genes that it expresses.”

When a traffic signal turns red it helps controls the flow of traffic. Similarly, inhibitory neurons help control the flow of electrochemical information. The type of information rosehip neurons control, and why they appear particular to humans, is yet to be discovered. “It has these really discrete connections with [pyramidal] neurons,” says Bakken. “It has the potential to sort of manipulate the circuit in a really targeted way, but how that influences behavior will have to come in later work.”

Found in the neocortex of human brains

The researchers identified rosehip neurons by looking at brain samples from two males who died in their 50’s and donated their bodies to science. The brain slabs were tissue from the neocortex, a most recent evolutionary development inside our skulls responsible for higher-order thinking. “The neocortex, the outermost layer of cells, is greatly expanded in humans–about a thousandfold compared to mice,”says Bakken. “From neurological studies, if you have a stroke in your neocortex for example, it really impacts your ability to do these sorts of high-order cognitive processing.”

The name “rosehip” came from Tamás and University of Szeged grad student Eszter Boldog. The two saw a resemblance between a rose that’s shed its petals and the cluster of axons around a brain cell’s center.

In the paper, Bakken and his colleagues note that the standard practice of studying human cognition by examining the brains of mice and other rodents, may need to be critically evaluated: “With the mouse cortex as the dominant model for understanding human cognition, it is essential to establish whether the cellular architecture of the human brain is conserved or whether there are specialized cell types and system properties that cannot be modeled in rodents.”

Bakken adds: “I would say at a basic parts level, we see a lot of similar parts between species… So model organisms are still very useful. But I think this study really points to the fact that if certain parts aren't there then you just obviously can't study them. I think it's really important to use model organisms, but keep in mind, you also need to study the actual tissue. That's why at the Allen Institute we're studying both human and mouse.”

What’s next?

The researchers will probe to see whether rosehip neurons are located in other parts of the brain. They will also look for rosehip neurons in postmortem brain tissue from deceased sufferers of neuropsychiatric disorders in an effort to note whether rosehip neurons play a role in human disease.

Living brain tissue

“It's pretty exciting,” says Bakken, his enthusiasm palpable. “We're doing an additional survey, a more wide and more in-depth survey of both the cortex, but also the whole human brain.” Bakken says they’re working with local neurosurgeons whose patients donate tissue that would otherwise be discarded after neurosurgery. “We're able to actually study living neurons from human brains and measure the electrical properties, the cells, their shape, their connections.” According to Bakken, the next project, and the goal of The Allen Institute as a whole, is to get a broader understanding of the diversity of the human brain.